Such monitoring processes are often used in gas turbines since combustion vibrations may occur during the combustion of gaseous and liquid fuels, which combustion vibrations may put components surrounding the combustion space into motion. Here, the components surrounding the combustion space are referred to as a combustor, wherein such combustion vibrations may occur both in the case of gas turbines with only a single annular combustor and in the case of gas turbines with a plurality of tube-shaped combustors distributed uniformly across the circumference. These tube-shaped combustors are referred to as “cans”. The acceleration sensors supply a signal which reproduces, firstly, the magnitude of the acceleration and, secondly, a frequency spectrum of the combustor accelerations.
In the prior art, the continuous time signal is processed by means of a Fourier transform at short time intervals—for example less than one second—to form a discrete frequency signal representing a corresponding time interval. The frequency spectrum of the processed signal established thus is then subdivided into a plurality of frequency bands and an individual threshold is set for each frequency band. A warning is displayed to the gas turbine operator provided that vibrations with an amplitude which exceeds the associated threshold occur within a frequency band and, if necessary, an emergency shutdown of the gas turbine is carried out so as to protect the combustors from vibrations damaging the component. A person skilled in the art refers to this shutdown as a “trip”.
Nevertheless, those accelerations of the combustor which do not trigger a “trip” may also lead to structure-mechanical damage. In the case of annular combustors lined with ceramic thermal shielding bricks, cracks may subsequently occur in the bricks, having a negative influence on the stability and integrity thereof. Moreover, consequential damages may occur, for example in the form of scaling on the support structure of the bricks as a result of hot-gas entry into the cracks.
It is for this reason that combustors, in particular bricked annular combustors, are subject to visual inspections during an inspection at regular time intervals. This should detect such defects at an early stage. If the defects are present, the damaged components or parts are subsequently replaced. However, the visual inspection of the gas turbine requires downtimes, which reduce the availability of the gas turbine.
It was found that visual inspections are also performed without any findings of damage being noted. In this case, the visual inspection was performed unnecessarily.